The invention falls within the field of data transmission. Although it can be used in principle in any digital communication system, the present invention and its underlying problem will be explained below with reference to a “ZigBee” communication system in accordance with IEEE 802.15.4.
So-called “Wireless Personal Area Networks” (WPANs) can be used for the wireless transmission of information over relatively short distances (about 10 m). In contrast to “Wireless Local Area Networks” (WLANs), WPANs require little or even no infrastructure for data transmission, so that small, simple, energy-efficient, and cost-effective devices can be implemented for a broad range of applications.
Standard IEEE 802.15.4 specifies low-rate WPANs, which are suitable with raw data rates up to a maximum of 250 kb/s and stationary or mobile devices for applications in industrial monitoring and control, in sensor networks, in automation, and in the field of computer peripherals and for interactive games. In addition to a very simple and cost-effective implementability of devices, an extremely low power requirement of devices is of critical importance for such applications. Thus, an objective of this standard is a battery life of several months to several years.
At the level of the physical layer, in the virtually globally available 2.4 GHz ISM band (industrial, scientific, medical) for raw data rates of fB=250 kbit/s, the IEEE standard 802.15.4 specifies a band spread (spreading) with a chip rate of fC=2 Mchip/s and an offset QPSK modulation (quadrature phase shift keying) with a symbol rate of fS=62.5 ksymbol/s.
In an 802.15.4 transmitter for the ISM band, the data stream to be transmitted is first converted to a sequence of PN sequences (pseudo noise) with the use of four data bits in each symbol period (TS=1/fS=16 μs), in order to select a PN sequence for a sequence set of a total of 16 PN sequences. Each symbol of four data bits is assigned in this manner a symbol value-specific PN sequence from 32 PN Chips (chip period TC=TS/32=500 ns=1/fC), which is transmitted instead of the four data bits. The sequence set of 16 “quasi-orthogonal” PN sequences, specified in the standard, in this case comprises a first group of eight first PN sequences, which differ from one another only by a cyclic shift of their chip values, and a second group of eight second PN sequences, which also differ from one another only by a cyclic shift of their chip values and from one of the first PN sequences only by an inversion of each second chip value (see IEEE Standard 802.15.4-2003, Chapter 6.5.2.3).
The PN sequences assigned to the successive symbols are linked together and then offset QPSK modulated (quadrature phase shift keying) by modulating, with half-sine pulse shaping, the even-indexed PN chips (0, 2, 4, . . . ) onto the in-phase (I) carrier and the odd-indexed PN chips (1, 3, 5, . . . ) onto the quadrature-phase (Q) carrier. To form an offset, the quadrature-phase chips are delayed by a chip period TC with respect to the in-phase chips (see IEEE Std 802.15.4-2003, Chapter 6.5.2.4).
Both coherent and incoherent approaches are known per se receiver-side to detect data symbols present in an incoming signal. Whereas in coherent approaches the incoming signal is converted into the complex envelope (baseband) with use of a carrier wave of the same frequency and phase and obtained from a carrier control circuit, in incoherent approaches at least the appropriates of the phase, within limits possibly also the appropriateness of the frequency of the carrier wave, can be dispensed with. Because of the higher realization cost in coherent approaches, which is also associated with an increased power requirement, in the current invention an incoherent receiver is used in which the incoming signal is converted at least not in-phase into the complex envelope and the resulting baseband signal is demodulated differentially.
Furthermore, it is known per se to multiply the data symbols from several communication participants transmit-side in each case with a participant-specific PN sequence from a sequence set of orthogonal PN sequences, to transmit the composite signal, and to detect receiver-side the data symbols of a specific participant in that the received composite signal is correlated with the PN sequence of this participant and subsequently decided. References is made in this regard, e.g., to the textbook “Nachrichtenübertragung” [Message Transmission] by Karl-Dirk Kammeyer, 3rd edition, B. G. Teubner, Stuttgart, ISBN 3-519-26142-1 (pages 632-635).